A Happy Family of Pachycephalosaurus

Distinguishing the skulls of juveniles and adults of the same species, and sometimes different species, can be a prickly thing in the fossil record. The result is that paleontology is littered with juvenile fossils that have been considered adults at some time or another. The crested duck-billed dinosaur Corythosaurus has also been known under names like Procheneosaurus, the famous Monoclonius is actually a juvenile of adult Centrosaurus, Styracosaurus, and kin, and the debate still continues on whether Nanotyrannus is a juvenile Tyrannosaurus.

Yesterday in the open access journal PLoS ONE, paleontologists Jack Horner and Mark Goodwin published a long-awaited paper positing synonymy for a trio of iconic (and melodiously-named) dinosaurs. The bone-headed dinosaurs Pachycephalosaurus, Stygimoloch, and Dracorex are all one and the same animal, according to their work. The latter two are juvenile stages, whereas Pachycephalosaurus represents a full adult.

Skulls of Pachycephalosaurus (top), Stygimoloch (middle; the front of the skull is missing), and Dracorex (bottom; the skull is crushed from top to bottom). In particular, note the changes in skull size and similarities in spike placement. Modified after the original in Horner and Goodwin 2009.

How is this possible? The animals look so different, right? Pachycephalosaurus has this big bowling ball on top of its head, which the other two lack. Stygimoloch has a uniquely-shaped, narrow dome, and Dracorex has a completely flat head. Furthermore, Pachycephalosaurus lacks the elongated spikes that make the other two look so fearsome.

Well, it turns out that this can all be attributed to ontogenetic changes (i.e., change as the animals get older). Horner and Goodwin assemble multiple lines of evidence for this hypothesis.

First, the skulls of Dracorex, Stygimoloch, and Pachycephalosaurus form a size gradation from smallest to largest--exactly what one would expect for a growth series. By itself, this is not irrefutable proof, of course--it could just be that Dracorex had a small adult size compared to Pachycephalosaurus.

Second, many of the knobs and bumps on the skulls can be matched up one for one between individuals of the various specimens. Alternatively, one would also expect that closely related (but different) species might have similar patterns of bumps. As Horner and Goodwin admit, there is some variation between individuals of the different "species"--but, the authors also note that this sort of variation is entirely expected and occurs even within undisputed adult Pachycephalosaurus.

Third, specimens of Stygimoloch, both in CT scans and physically cut specimens, show an open suture between the two frontal bones of the dome. Pachycephalosaurus domes are completely fused up. Open sutures are often strong indications that an animal is still growing--and, it's particularly intriguing that a small "species" has them but a large "species" doesn't!

Finally, microscopic examination of the bones in two of the three "species" (Stygimoloch and Pachycephalosaurus; there weren't any Dracorex available for cutting up) shows that Stygimoloch was still growing (and thus not a full adult)--but Pachycephalosaurus specimens weren't growing much at all (and therefore were probably full adults).

Any one of these lines of evidence might be interesting, but not completely convincing. Taken together, however, they make a pretty compelling case that Dracorex and Stygimoloch are juvenile Pachycephalosaurus. Because Pachycephalosaurus was named first, the first two become junior synonyms. It's a shame, because they're such cool names!

As for duckbilled dinosaurs, horned dinosaurs, and even modern crested birds like the cassowary, the story in the pachycephalosaurs suggests that weird ornaments on the skull were something that happened only as the animals approached full size. The domes practically appeared overnight! The teenage years must have been a real headache for these dinosaurs.

Thanks to the wonders of open access, the article is freely available for all to read. Additionally, it is worth taking advantage of the rating and comment features at PLoS ONE [disclaimer: I am a section editor for that journal]. . .few other scientific publications allow the readers to annotate the papers directly!

Citation
Horner, J., & Goodwin, M. (2009). Extreme cranial ontogeny in the Upper Cretaceous dinosaur Pachycephalosaurus. PLoS ONE, 4 (10) DOI: 10.1371/journal.pone.0007626